feat: email notifications + deadline reminder system

Database:
- notification_preferences table (user_id, tenant_id, reminder days, email/digest toggles)
- notifications table (type, entity link, read/sent tracking, dedup index)

Backend:
- NotificationService with background goroutine checking reminders hourly
- CheckDeadlineReminders: finds deadlines due in N days per user prefs, creates notifications
- Overdue deadline detection and notification
- Daily digest at 8am: compiles pending notifications into one email
- SendEmail via `m mail send` CLI command
- Deduplication: same notification type + entity + day = skip
- API: GET/PATCH notifications, unread count, mark read/all-read
- API: GET/PUT notification-preferences with upsert

Frontend:
- NotificationBell in header with unread count badge (polls every 30s)
- Dropdown panel with notification list, type-colored dots, time-ago, entity links
- Mark individual/all as read
- NotificationSettings in Einstellungen page: reminder day toggles, email toggle, digest toggle

backend/internal/models/notification.go

@@ -0,0 +1,32 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/lib/pq"
+)
+
+type Notification struct {
+	ID         uuid.UUID  `db:"id" json:"id"`
+	TenantID   uuid.UUID  `db:"tenant_id" json:"tenant_id"`
+	UserID     uuid.UUID  `db:"user_id" json:"user_id"`
+	Type       string     `db:"type" json:"type"`
+	EntityType *string    `db:"entity_type" json:"entity_type,omitempty"`
+	EntityID   *uuid.UUID `db:"entity_id" json:"entity_id,omitempty"`
+	Title      string     `db:"title" json:"title"`
+	Body       *string    `db:"body" json:"body,omitempty"`
+	SentAt     *time.Time `db:"sent_at" json:"sent_at,omitempty"`
+	ReadAt     *time.Time `db:"read_at" json:"read_at,omitempty"`
+	CreatedAt  time.Time  `db:"created_at" json:"created_at"`
+}
+
+type NotificationPreferences struct {
+	UserID              uuid.UUID     `db:"user_id" json:"user_id"`
+	TenantID            uuid.UUID     `db:"tenant_id" json:"tenant_id"`
+	DeadlineReminderDays pq.Int64Array `db:"deadline_reminder_days" json:"deadline_reminder_days"`
+	EmailEnabled        bool          `db:"email_enabled" json:"email_enabled"`
+	DailyDigest         bool          `db:"daily_digest" json:"daily_digest"`
+	CreatedAt           time.Time     `db:"created_at" json:"created_at"`
+	UpdatedAt           time.Time     `db:"updated_at" json:"updated_at"`
+}